Natural user-input (NUI) technologies aim to provide intuitive modes of interaction between computer systems and human beings. Such modes may include gesture and/or voice recognition, as examples. Increasingly, a suitably configured vision and/or listening system may replace traditional interface hardware such as a keyboard, mouse, touch-screen, gamepad, or joystick controller, for various computer applications.
A function of any user-input technology is to detect user engagement—i.e., a condition wherein a user desires to provide input. With traditional interface hardware, user engagement is trivial to detect: every key press, screen touch, or movement of the mouse or joystick is an indication that the user desires to provide input. With NUI technologies, however, detection of user engagement is frequently not trivial.